With the observation of high-energy astrophysical neutrinos by the IceCube
Neutrino Observatory, interest has risen in models of PeV-mass decaying dark
matter particles to explain the observed flux. We present two dedicated
experimental analyses to test this hypothesis. One analysis uses six years of
IceCube data focusing on muon neutrino 'track' events from the Northern
Hemisphere, while the second analysis uses two years of 'cascade' events from
the full sky. Known background components and the hypothetical flux from
unstable dark matter are fitted to the experimental data. Since no significant
excess is observed in either analysis, lower limits on the lifetime of dark
matter particles are derived: We obtain the strongest constraint to date,
excluding lifetimes shorter than $10^{28}\,$s at $90\%$ CL for dark matter
masses above $10\,$TeV.